Elastomer boots are primarily used for sealing two parts that can be articulated relative to one another and which, more particularly, rotate at the same time. These parts constitute a joint. A typical application refers to sealing joints of the constant velocity and universal types. For this purpose, a boot with a cylindrical portion, typically having a smaller diameter is slipped on to a shaft connected to a first joint component, and an annular portion with a greater diameter is connected either directly or via an intermediate element to a second joint component. Between the cylindrical portion mentioned first and the annular portion with the greater diameter, there extends a wall. The wall has the shape of half a torus for a roll boot and has a bellows shape for a convoluted boot. When the two joint components carry out an articulation movement relative to one another, the radius of curvature of the wall decreases on the inside of the angle and increases on the outside of the angle. When the joint rotates in the articulated condition, the change in curvature in the roll boot wall moves across the circumference, so that during a complete 360° rotation, each point of the boot wall passes through a curvature maximum and a curvature minimum causing flexing of the boot wall. Flexing also occurs for each rotation of the boot due to gravitational and centripetal forces.
The boot is subject to internal flexing work that can lead to an increase in the temperature of the boot. The increase in temperature causes the boot material to be subject to an increase in chemical and thermal attack and may accelerates the boot degradation. Also, the increase in temperature degrades the material of the boot wall at an accelerated rate due to the increased chemical reaction caused by the exposure of the boot seal with the external and internal fluid environments.
The material of the boot wall also degrades because of fluid contact at the external and internal wall surfaces. The external surface of the boot is in contact and exposed to atmospheric conditions subjecting the material to the effects of weathering, chemical and other forms of degradation. The internal surface of the boot is in contact and exposed to joint grease subjecting it to chemical and other form of degradation. Selecting a material that is suitable for both environments can minimize the degradation in the boot wall.
Also, the intense flexing of the boot subjects the material to stress. If the boot was in an unstressed situation, a film layer of protection could remain on the material, such as with rubber. Without the film layer, the material is more prone to attack by chemical reaction and elastomer bond rupture caused by environmental exposure to elements, such as to ozone and oxygen. One form of bond rupture leads to an effect known as scission. Scission is a form of degradation where the material bonds are broken along polymer chains and results in a tearing type effect rendering the boot as an ineffective seal. Other harmful effects may occur if the material is selected improperly for the particular environmental exposure leading to decrease in mechanical strength and sealing effectiveness.
Boot construction is typically from a single material. The material is selected to provide the best protection from external and internal environmental exposure while providing the best mechanical properties for the application. By selecting a material that is more suitable for either the external or the internal environment, it leaves the one surface of the material open to less than optimal protection from its environment. The other choice is to pick a material that is somewhat resistant to both external and internal environments. However, this is not ideal because it leaves both surfaces of the boot open to attack and degradation. It would be beneficial to have a way to protect the external and internal surface of a boot wall from degradation when both surfaces are continually or intermittently exposed to different fluids.